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How Special Relativity Makes Magnets Work



MinutePhysics on permanent magnets: http://www.youtube.com/watch?v=hFAOXdXZ5TM
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Magnetism seems like a pretty magical phenomenon. Rocks that attract or repel each other at a distance - that's really cool - and electric current in a wire interacts in the same way. What's even more amazing is how it works. We normally think of special relativity as having little bearing on our lives because everything happens at such low speeds that relativistic effects are negligible. But when you consider the large number of charges in a wire and the strength of the electric interaction, you can see that electromagnets function thanks to the special relativistic effect of length contraction. In a frame of reference moving with the charges, there is an electric field that creates a force on the charges. But in the lab frame, there is no electric field so it must be a magnetic field creating the force. Hence we see that a magnetic field is what an electric field becomes when an electrically charged object starts moving.

I was inspired to make this video by Prof. Eric Mazur http://mazur.harvard.edu/emdetails.php

Huge thank you to Ralph at the School of Physics, University of Sydney for helping us out with all this magnetic gear. Thanks also to geology for loaning the rocks.

This video was filmed in the studio at the University of New South Wales - thanks to all the staff there for their time and support.

Music: Firefly in a Fairytale, Nathaniel Schroeder, and Love Lost (Instrumental) by Temper Trap licensed from CueSongs.com


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Runtime: 4:19
Comments: 4045

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Comments:

Author breakingthemasks ( ago)
never commented here before...but this video and it's twin were stellar. more high level videos! great job🖒

Author Crafty DIY Craftlantic ( ago)
what if the positively charged cat moved the one way when the negatively charged atoms in the wire went the other way ?

Author Alexander Dräbenstedt ( ago)
So does an electron beam, which is a current without surrounding protons, not produce a magnetic field? I would doubt that.

Author Ryan Koepke ( ago)
what is the positive "cat" was moving near the speed of light? What would that do to the charge?

Author makro80 ( ago)
How does the cat near the neutral wire at 3:00 get pushed away? Like he said in the video its due to the magnetic field around the wire but I dont understand how that would cause it to be repelled?

Author nikitha nayaer ( ago)
mind = blown
oh and how did you assume the cat was female what if it identified it's self as a helicopter

Author Cat Man ( ago)
The best explanation I've seen for electromagnets

Author Engineer Ahmad ( ago)
Direction of Magnitic field is tangent to conductor not away from it

Author Kyle MacDonald ( ago)
How do you grow such a thick beard in six hours? Takes me a week and a sacrifice to the eldritch follicular powers.

Author peanutaxis ( ago)
The velocity of an electron in a 2mm radius wire with 1 ampere flowing throuogh it is:

0.000023m/s

.....and I'm supposed to believe that relativistic effects are relevant. I don't *&%ing think so.


wiki: Drift_velocity#Numerical_example

Author erikk77 ( ago)
Thank you Michael Faraday.

Author Dercio Silveira ( ago)
I dreamed that a moving charged particle affect some quantum field like plane´s wings, creating lifting (magnetisim)...

Author Cold Death By Multiverses ( ago)
This all sounds more like a classic interpretation rather than a relativistic one, because only Galilean Relativity was used to explain it.

Author deltango12345 ( ago)
cool!

Author Mike Billars ( ago)
This doesn't make sense to me. I always thought that electrons actually move *very* slowly through a conductor - like millimeters per second. If that's true, then the Lorentz contraction would be so insignificant that I can't imagine it having any effect at all. What am I missing?

Author Lu Chee Ket ( ago)
What consequent can we tell when a high power transformer is performing in a strong electrostatic field?

Author Lu Chee Ket ( ago)
Right hand screw rule is best to rely and consequently charges appear with relative motion induce equal charges to flow in opposite direction as equivalence to magnetic's in or out for determination of force direction.

Author Lu Chee Ket ( ago)
The prevailed +dQ to moving +q repels, and the prevailed -dQ to moving +q attracts with opposite current seems to be logically right.

Author AvioPhile ( ago)
That is pretty, then in all reference frames we agree on the end result. How does it fit with electromagnetic waves?

Author Eliyahu Kassorla ( ago)
It's because cats have pos. Like a cation being positive.

Author Jari Skogster ( ago)
There is no magnetic force between still standing ions and a moving cat. Still standing charges don't have a magnetic field. You explained a thing that does not exist!

Author frank x ( ago)
BUT electrons in a wire are moving AT A SNAIL'S PACE, (my typing fingers are more time-dilated, relative to the KB than the electrons that shunt a bit when the little connections are made when keys are hit). Care to explain?

Author MidnightSt ( ago)
when i saw the title of this video, i was like "GOD, YES!"
Awesome. First real explanation of (electro)magnetic fields i ever got. Thank you very much, have an awesome life! :)

Author M Goudsmits 金马桥 ( ago)
actually you should be able to test it
th ecat only needs to do about 30 cm per second

Author lynn rathbun ( ago)
NO----passing a current through a metal does not make IT a magnet. It makes a magnetic field in space but it does not do anything to the metal.

Author ThunderboltTangerine ( ago)
Whats the song near the end?

Author Markus Helenius ( ago)
love ur work
<3

Author DrDress ( ago)
What!? Is this for real? I've never heard of this. That's amazing!

Author Venkatesh Babu ( ago)
Most of the elements emit some kind of waves because of the properties.

Author IqbalHamid ( ago)
Who believes this? Yes, it's an interesting idea, BUT it feels contrived. Furthermore, as suggested by the video, the maths don't seem like they would add up. I refer to the speed of the electrons in the wire. Another consequence of this theory which sits uncomfortably with me is that the force on the cat suggests an ASYMETRY between observers. Meaning: in the frame of reference of the stationary observer, there should be no force! He should witness no force! But he does. Hence the feeling of it bring contrived. Is there any experimental or other evidence to support or suggest this theory. Still, it is better than no theory.

Author Alex Botsford ( ago)
So the faster the cat moves, the greater the magnetic repulsion?

Author Luis Augusto Libório ( ago)
O_O

Author Shawn Strickland ( ago)
30,330 likes and 330 dislikes, I want to like this but my OCD won't let me

Author Caleb Griffith ( ago)
I can't stand Minute Physics narrators voice.

Author Quintin Hansen ( ago)
where do the north and south come from

Author Billy Willy ( ago)
Or, like Richard Feynman put it, magnetism is the relativistic aspect of the electric field. But there is really only the electric field. There's also an analogous "magnetic" component of the gravitational field. It's called gravitomagnetism and was measured by the famous Gravity Probe B satellite experiments. Google "Gravitomagnetism".

Author Joaquín Herreros ( ago)
i don't understand the explanation: if a charge is moving linarly in one direction, the electric field and the magnetic field are perpendicular between them and both also with the direction of movent ("right hand law"), so here are saying that the magnetic field is a relativistic efect of an electri field, saying somehow that are only electric fields involved in the right hand law?????

Author Alexander Zalesov ( ago)
I'm a bit sceptic about this explanation.

Author Lg Ft ( ago)
I don't wanna talk to no scientist, yall motherfuckers lying and gettin me pissed

Author RODLON ( ago)
Do really electrons drift through wires at 3cm per second?

Author Prasad NSG ( ago)
You started to clear the fog of magnetism in my brains!!!!!!!!!!!!!!!!!!

Author Jose Lanza ( ago)
somebody is getting animation graphic designer money

Author RSGTomcat ( ago)
I've always wondered... if the electrons are moving in your frame of reference (if the cat is also stationary to you) wouldn't they take up less space (and contract) due to special relativity and attract the non moving cat?

Author Jonathan01590 ( ago)
No way you can grow that beard in 6 hours

Author John Coppola ( ago)
Albert Einstein said: "The special theory of relativity owes its origins to Maxwell's equations of the electromagnetic field." This viewpoint seems a bit like a circular argument to me. Cool you can explain magnetism this way but the linear Maxwell's equations are a more elegant way to explain it. Can't visualize circular lines of force with the relativity explanation of magnetism really just being moving charges. I think this effect is just a correction term.

Author Nick Flock ( ago)
so wait, when the cat is stationary, is case a:"the fact that the electrons are moving through the wire cause the space between them to contract, therefore attracting the stationary positive cat" true or case B: "The electrons are moving and do shrink but the space is not moving so they stay the same distance apart and therefore do not affect the net charge of the region." Because I could see reason for both. Case A: why else would the electrons move apart as the reference frame begins to equalize speed. no Net difference of movement should show the particles true, unaffected and un-stretched formation and the fact that equalizing speed caused them to spread out leads me to believe that there was contraction when the cat wasn't moving. But case B seems plausible as well because if the electrons were being contracted then one would see attraction between the cat and the wire, yet he clearly stated that there was no attraction between the two.

1:if one case is correct, then please explain why the other, OR BOTH, is wrong.
2:if B is correct, then how do electromagnets attract stationary objects?
3:also, regardless of case, does that mean an electromagnet's strength and even charge depends on how fast you are moving relative to the electrons. For example would the cat traveling half of the electrons speed see more proton contraction and [possibly] less electron contraction cause a weaker force on the cat than if the cat were traveling the speed of the electron, which (in this frame of thinking) cause more proton contraction and [possibly] less electron contraction.

Author Saint Mendez ( ago)
Dead wrong, magnetism doesn´t work like that. besides, the electrons inside a conductor move about 0.5 mm/s at about 120 v. The magnetic field is produced by alignment of the electrons in a circular motion when being pulled by an electric field opposite to the currents direction. Relativity has no part on it.

Author cody x ( ago)
if we are talking about a non moving cat, wouldn't the electrons be more compressed than when they were still? the non moving cat should be attracted then, right?

Author Jack Specht ( ago)
If this hypothesis were true it would seem that the force of electromagnetism would be calculable in some manner of proportionality to the theoretical time dilation described in special relativity . Since time dilation is described by special relativity as infinitesimally small it would seem that the force origination , generated and described by hypothesis would be negligible even if it were an exponential proportion .

Author Raymond K Petry ( ago)
In the static state the positive-cat draws wire-electrons, setting up a mirror wire image of negative charge drawing them together... In flowing, the wire-electrons speed up, nearing the positive-cat, avoiding and repelling it... Were electrons directly, causing, the magnetic field there'd be no current through capacitors—so—current is charge-displacement-flow...

Author Raymond K Petry ( ago)
[01:09] "positive metal ions" only if the wire is vapor heated to be an electroplasma...
[02:00] 'contracted separation' of protons—but—not previously [01:32] of electrons...

Author cole royer ( ago)
this video brings me back

Author markdude100 ( ago)
There is an error in this video. @2.28 You say that the stationary feels no force; however, in the cat's frame of reference the electrons will appear to be contracted. Therefore, a net negative charge is created, so the cat is then attracted.

Author François Cauneau ( ago)
Wow !! That's teaching !! Congratulations !!

Author Alex Shtyrlin ( ago)
Shouldn't the magnetic force push the not-moving positive particle only besause of the lenght contraction of the electrons?

Author MindlessTurtle ( ago)
that was like watching someone force a square peg in a round hole: you can make it fit, but it doesn't make sense to do that

Author Frank Harr ( ago)
And here I thought cat ions would be negatively charged.

Oh! And 0.000 000 000 100% c=
0.00108 km/h,
0.000300 m/s,
0.000984 fps,
0.000671 mph and
0.000583 kn.

I can crawl faster than that.

I guess the force that propels the electrons acts faster than the actual movement of the electrons. That is, electrons push each other away faster than they move. Like a block of wood.

Also, I gather that the effects of SR have some kind of scaling to do with the nature of the system. So its more noticable, noticible, noticeble, observable in electrons and less so in slowly pitched softballs.

Author Mohammed Benaissa ( ago)
only in your frame of reference bitch , lol 0:54

Author James Klinger ( ago)
I'm going to have to watch this more than once lol

Author Mita Binderti ( ago)
If the electrons are moving from my frame of reference, shouldn't they contract causing a negative charge on the wire thus attracting the cat if it were stationary?

Author viralshield ( ago)
a hypothesis only !

Author klaasbil ( ago)
Wow, I have never heard of this explanation but it does make sense. I tend to think that Veritasium is smart enough to have come up with this originally, but at the same time I think this is unlikely and it must have been postulated before. Kudos for explaining it so well though - as always.

Author Sana ullah ( ago)
if any body among the viewer has university level degree then help me can we trust these videos

Author seedstyl ( ago)
I'm confused...electro magnets work on stationary pieces of iron with no "moving" charges, so how does the analogy work in real life?  Does this mean that the moving positive charges in the wire create movement of the positive cat and in turn also repel the cat?  I think the cat thing threw me off haha.

Author Jason McAdam ( ago)
Where can I get more specifics on the mathematics of this all?

Author saurabh bagade ( ago)
I have a doubt though why isn't the cat attracted while it is stationary?

Author akash patel ( ago)
That's interesting .i don't know that

Author gecomert ( ago)
3:23 vsauce?

Author Vishal Upendran ( ago)
Beautifully explained! You have managed to beautifully illustrate and abstract the Lorentz invariance in a sleek manner.
Kudos!

Author Tyler Mauldin ( ago)
This is awesome! Great video!

Author Jason Soto ( ago)
So if can get charged particles to move at lets say 1m/s i can make a ridiculously powerful magnet? What if we make a plasma, separate the charged particles and spin that in some kind of centrifuge? i'm sure we can get electrons moving much faster than .03 m/s :D

Author Nathaniel Flear ( ago)
But if the protons get denser where do the extra protons come from?

Author Leonardo Rumalean ( ago)
Why won't the cat be attracted to the wire whilst not in motion.
Shouldn't it be?
After all there should be length contraction from the cat's frame of reference so there should be a higher density of electrons. This is if there is a current flowing by the way.

Author Robert Strauss ( ago)
From Derick's frame of reference, the electrons moving and the cat and protons not, the electron's should experience length contraction relative to him and the cat. This would mean (using the same logic as in the video) that the electrons are now closer together, and there are more in a length of wire, creating an average negative charge in the wire, and pulling the positively charged cat towards the wire. Right?

Author Attila Kiss ( ago)
Funny story, that we asked the same in high school from teacher, where the Lorentz force is going if the frame of reference has been put on moving charge. Actually she could not answer.
Actually Einstein discovered the special relativity in this way, he noticed the non-relativistic approach causes contradictions in the electromagnetic phenomena.

Author Firaro ( ago)
So electromagnets dont work if the piece of nickel stays still?
Im not saying your explanation is wrong, but i am saying i dont understand it. And it doesnt make sense since electromagnets work on unmoving objects. So please, make a more clear version, or just leave a clarifying comment. I want to know

Author Kata Mae ( ago)
Very confusing video that takes poeple. The question keeps poping up :
why doesn't moving negative charges shrink ?
I don't know for sure but I think of a simple reason :
Negative charges are not connected to each other, they move freely, independently, contrary to positive charges. If they move, they can space themself up or down to keep neutrality. Positive charges don't, they move each other stuck in their own space and the space is shrinking.
Or maybe there is some global effect of quantum mechanics at stake...

Author Quoutub ( ago)
When he said cat, I thought he meant cadmium ^^

Author Marcus Tornea ( ago)
Something isn't clear to me here. No, it's not the issue about electrons being length contracted and attracting the positive cat-ion, but look at a permanent magnet. No current runs through a permanent magnet, the magnetic field around a permanent magnet is produced because the electrons, which act like little magnets of their own due to the fact that they spin around their own axis, line up to make a majority magnetic domain. The electrons in the permanent magnet are not length contracted but still do exert an electromagnetic force on charged particles. What is this?

Author Jee Whizz ( ago)
my mind is totally blown!

Author Daniel Skyminer ( ago)
At 1:37, wouldn't the electrons undergo length contraction due to special relativity in the cat's frame of reference (stationary relative to the protons)? Therefore there would be more electrons and the cat would be attracted to the rod, right? What am I missing?

Author TheLegend27 ( ago)
59:9.0999999 nine nine nei nein nein
woah
Hitler says nein
Hitler is evil
i cant rhyme
but this is devil

Author Tarun Chand ( ago)
what about the stationary cat! for the stationary cat should see the electrons contract thus increasing the density of -ve charge. the stationary cat should be attracted! why does it not happen?

Author econogate ( ago)
Everytime people try to define magnetism they always have to refer to gravity and relativity, gee I wonder why? Could it be...naw. We could be piloting crafts with bursts of magnetism but that maybe too much for physicists heads to absorb yet.

Author Vijay Raj ( ago)
OK so that explains magnetism, essentially it's electric field and electric field only.

Now how do you explain a electromagnetic wave? There is no charged particles there right?

Author To Athánatos ( ago)
Well, in my perspective, that cat is utterly repulsive.

Author Tomek ( ago)
How does it relate to amper's law? Current flowing in the same direction in two paralel wires causes them to attract each other. Since moving electrons are causing both wires to be positively charged shouldn't the repel each other???!!!

Author Christian Libertarian ( ago)
Unbelievable. I have learned something. Hasn't happened in years.

Author Sad Man ( ago)
Explanation needed:
If positively charged cat is moving, then even without current there should be some length contraction. (one half of when both the cat and the current are moving).
Does this mean that there is magnetic force even if there is no current as long as the positively charged particle is moving?

Author Yout Tuka ( ago)
I just learned this in physics lecture. Pretty cool!

Also, you can derive the speed of light by setting the electrostatic force equal to magnetic force.

Author Dimitris Davazoglou ( ago)
And how can we explain the mutual attraction between two parallel electron beams, for instance? Now there are no immobile positive ions, just moving negative electrons.

Author Purely Science ( ago)
electro MAG-NETism means NET MAGnitude of electrons.

Author Purely Science ( ago)
that's why MAG-NET means NET MAGnitude of charges.

Author Mazahir Mammadli ( ago)
Name me a textbook that explains this topic even half as simply as Veritasium. Derek, you're a legend!

Author brianjlayman ( ago)
i came here from minute physics :) love both u guys and vsauce

Author SaintDank ( ago)
What if the cat wasn't moving at the same speed as the electrons

Author Infinity 1 ( ago)
100 × pi dislikes

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